implements solution

main.go

@@ -1,10 +1,10 @@
 package main
 
 import (
+	"container/heap"
 	"fmt"
 	"math/rand"
 	"sort"
-	"sync"
 	"time"
 )
 
@@ -34,23 +34,64 @@ func source(c int) <-chan int {
 }
 
 func merge(cs ...<-chan int) <-chan int {
-	wg := sync.WaitGroup{}
 	out := make(chan int)
 
-	output := func(c <-chan int) {
-		for n := range c {
-			out <- n
-		}
-		wg.Done()
-	}
-
-	wg.Add(len(cs))
-	for _, c := range cs {
-		go output(c)
-	}
 	go func() {
-		wg.Wait()
+		h := &IntHeap{}
+		heap.Init(h)
+
+		// prime the pumps
+		for i, src := range cs {
+			head, ok := <-src
+			if !ok {
+				continue
+			}
+			it := item{
+				val: head,
+				src: i,
+			}
+			heap.Push(h, it)
+		}
+
+		for h.Len() > 0 {
+			top := heap.Pop(h).(item)
+			out <- top.val
+			if head, ok := <-cs[top.src]; ok {
+				it := item{
+					val: head,
+					src: top.src,
+				}
+				heap.Push(h, it)
+			}
+		}
+
 		close(out)
 	}()
 	return out
 }
+
+// An IntHeap is a min-heap of ints.
+type IntHeap []item
+
+func (h IntHeap) Len() int           { return len(h) }
+func (h IntHeap) Less(i, j int) bool { return h[i].val < h[j].val }
+func (h IntHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
+
+func (h *IntHeap) Push(x interface{}) {
+	// Push and Pop use pointer receivers because they modify the slice's length,
+	// not just its contents.
+	*h = append(*h, x.(item))
+}
+
+func (h *IntHeap) Pop() interface{} {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[0 : n-1]
+	return x
+}
+
+type item struct {
+	val int
+	src int
+}